1. Field of the Invention
The present invention relates to solid-state image pickup apparatuses used in scanners, digital still cameras, digital camcorders, etc.
2. Description of the Related Art
In recent years, CMOS image sensors have been widely used in digital still cameras, digital camcorders, and camera units included in mobile telephones. A technique for integrating A/D conversion circuits into a CMOS image sensor has been developed so as to reduce the number of components and power consumption. Various types of CMOS image sensors including A/D conversion circuits have been proposed, and include a CMOS image sensor with an A/D conversion circuit provided for each column of pixels called column A/D conversion circuits.
Such an A/D conversion circuit compares an analog signal with a reference signal. Japanese Patent Laid-Open No. 2005-348324 discloses a technique for applying a voltage having a fixed value to A/D conversion circuits as a reference signal. In this case, since a constant current passes through both ends of a resistance element included in each of A/D conversion circuits that are individually provided for columns, it is possible to more easily enhance speed as compared with a case in which a reference voltage is changed with time.
FIG. 10 is a schematic diagram illustrating a configuration of a solid-state image pickup apparatus in the related art, and cites FIG. 1 of Japanese Patent Laid-Open No. 2005-348324. For explanation, reference numerals different from those used in Japanese Patent Laid-Open No. 2005-348324 are used.
A reference voltage V_DE having a fixed value is input into A/D conversion circuits that are individually provided for columns. When these A/D conversion circuits start a conversion operation, a constant current passes through both ends of a resistor R included in each of the A/D conversion circuits. However, since a line CL for supplying a reference voltage having a fixed value has a resistance value, a potential gradient occurs in the line CL. That is, since a voltage across both ends of the resistor R differs from column to column, a current passing through both ends of the resistor R differs from column to column.
Even if integration operations are individually started in columns at the same time and the values of analog signals to be converted into digital signals in the columns are the same, a period of time used for inversion of the outputs of a comparator COMP may differ from column to column because the value of a current passing through the A/D conversion circuit differs from column to column. Accordingly, shading occurs in a direction along a single row from which signals that have been converted at the same time are output. If the number of columns of pixels in a solid-state image pickup apparatus and the length of a line for supplying a reference voltage are increased, this phenomenon becomes more pronounced.